1. Field of the Invention
The present invention is related to apparatus for viewing or recording of narrow frequency band images at selectable center frequencies over the broad frequency band from optical through the far infrared or thermal frequencies, and more particularly to apparatus for the described purpose incorporating therein storage means and utilizing composite cathodes.
2. Description of the Prior Art
Ordinarily, expensive (multimillion dollar) optical camera systems are utilized for recording the best quality optical images on film of aircraft, missiles, and the like, at extended ranges. Such equipment and procedures are typically used for purposes of measuring attitude and obtaining detailed information concerning events or characteristics of the objects. In spite of the very good relative quality of the images, they often are not good enough for the extraction of needed data, however. Inasmuch as photographic film does not directly respond to longer wavelengths and to their lower photon energy, such optical systems cannot be used for infrared or thermal imaging.
Photographic film is typically capable of image resolution in excess of 500 line pair per mm. For illustrative purposes, a large telescope may ordinarily furnish images having resolutions in the range of 80 line pair per mm. Accordingly, 35 mm. film, a typical film for optical systems, furnishes approximately 2,800 line pair resolution at 80 line pair per mm. As a contrast, a typical television system can furnish 262 line pair resolution.
Thermal Imaging Systems typically use scanning techniques. That is, the system optically scans a solid angle of space and develops the final image sequentially, or point by point. Some thermal scanning systems are available with resolution characteristics which approach the characteristics of television systems, but at costs of 50 to 100 times that of television. Sequential scanning systems incorporate substantial deficiencies, however, inherent in their sequential nature. One such deficiency is the ability to view only one point, or a relatively small number of points, in the field of view at any one time. Thus, an event occurring between scans is neither detected nor imaged. For example, a scanner of good TV quality resolves approximately 272,000 picture elements. Where such a scanner views but one element at a time, it will miss what happened at the other 271,999 elements. This is a serious problem where dynamic objects are of interest, or where the objects or events of interest are either relatively small in size or short in duration. For example, an object which is one meter or less in size, at 16 KM distance, in a 36 mr field of view would always appear to be at least one to two meters in size, if detected at all. Moreover, if the duration of the activity of interest is less than a full frame scan time, it may not be detected at all. Thus, if the above example utilizes a scanner having an imaging rate of 30 frames per second, the dwell time on each picture element or picel is 1/30.div.272,000, or 1.225.times.10.sup.-7 second. Such a dwell time is very short, leading to requirements for a highly sensitive and very low noise detector and a high gain amplifier. If better optical quality is desired, as is available from 35 mm. film for example, the problem is compounded by a factor of (2,800/262).sup.2 or 114, the square of the ratio of the line pair resolutions. For this level of optical quality, the dwell time becomes only 1.07 nanosecond per picel.
This problem can be lessened by adding more detection-amplifier channels. The approach of using multiple point scanners, or multiple detection amplifier channels, provides more picture elements per unit time, but simultaneously complicates and increases the cost of what is already a complex and expensive system.
Another attempt at thermal imaging is the pyroelectric vidicon, which offers the use of full frame dwell time. However, in the pyroelectric vidicon the thermal image quickly spreads through the pyroelectric and backing material by heat flow, and image detail is lost. Thus far the vidicon, while typically less expensive than the scanner, has not obtained images of scanner quality.
In the television imaging art, charge coupled devices (CCD) are being used to replace vidicons under some circumstances. New CCD systems are being developed for use with thermal wavelengths, but considerably more progress is needed to make them practical. Such devices may produce images having "TV" quality, which is inadequate for dynamic target imaging in many cases. A substantial state-of-the-art increase is needed if good thermal imaging of dynamic targets is to be accomplished.
There are urgent requirements for thermal imaging systems which can operate at 100 frames per second, obtain optical quality comparable to that available from 35 mm. film (more than 3.14.times.10.sup.7 picels), and good contrast (greater than 1,000 to 1). The thermal imaging systems now in development offer little hope for ever meeting this type of performance.
Several problems of the prior art must be overcome in order to provide a good thermal imager:
1. The thermal image must be sensed and processed as an image, rather than as one or several picels at a time.
2. The image must be sensed and transferred into electronic storage in time sufficiently short to insure very little image smear. Times less than 100 microseconds are typically desirable.
3. Many very short time-sampled images must be integrated in electronic storage so as to enhance the image to noise ratio.
4. Very narrow band thermal spectra are typically required and the desired spectrum should be quickly and easily selectable.
5. Recording on continuously moving film, as opposed to the standard process utilizing stop action pin registration for framing, is highly desirable.